For a ring laser gyroscope, hereinafter RLG, it is well known that there are two laser beams propagating in opposite directions within a cavity of the RLG. Any frequency difference between these two light beams represents an inertial rotation. In order to avoid the problems of the well-documented lock-in effect, the RLG may be dithered about its axis. For a three-axis RLG assembly, it has been taught in U.S. Pat. No. 4,477,188 by Stiles, et al. that the three RLGs in the assembly can be dithered about an axis equally shared by all three RLGs. Dithering, as was pointed out, is an angular vibration imposed upon the body of the RLG and is a well-accepted means of mitigating the lock-in effect.
To perform the dithering, conventionally a dither spring suspension mechanism, which includes a plurality of transducers, is used. These transducers are made up of piezoelectric elements which, as is well known, are "bilateral"--being able to convert an electrical signal into a physical movement and, conversely, convert a physical movement into an electrical signal. To ensure that the rate at which the RLG is dithered is of the proper amplitude, a means for amplitude control is used. Currently, to obtain a measure of the dither rate, one or two of the transducers in the dither spring suspension mechanism are used as sensors for detecting the motion of the RLG body, since, as was discussed earlier, these transducers are "bilateral" and thus can be used both as sensors and actuators. When used as a sensor, the transducer is known as a pickoff element. When used to dither, the transducer is known as a torquer element.
Yet problems do exist for the prior art feedback system. The most notable of the problems occurs when the pickoff transducer element fails. In the case of a single pickoff transducer feedback system, when the single pickoff transducer element fails, the RLG no longer operates, for not only does the suspension system not have a pickoff signal for providing the regeneration required to maintain the system in an oscillatory condition, but also no measure of rate is available.
For a feedback system which has two pickoff transducer elements, when one of the pickoff elements fails, half of the feedback signal is lost. Consequently, the control system responds by driving the torquer elements with a larger signal in attempting to restore the pickoff signal to the initial amplitude. If it is capable of such overdrive, the system will end up with twice the desired amplitude of dither motion. Often, however, large amplitudes have a deleterious effect not only on the performance of the instrument but also on the ability of the electronics to sustain the overdrive, or of the driven transducers to survive. Similar scenarios can be constructed for cases using three, and more pickoff elements.
So far as known, a maximum of two pickoff transducer elements is used. This is due to the fact that were more of the transducer elements used as pickoff sensors, proper dithering of the RLG becomes difficult, as there no longer would be a sufficient number of transducers to dither the RLG, since dithering results from a summation of the deformation of the different torquer transducer elements, in response to a driving signal.